1. Field of the Invention
The present invention relates to an induction heating apparatus, and more particularly to an induction heating apparatus being provided with an unsuitable load detecting circuit and used for household cooking.
2. Description of the Prior Art
The induction heating apparatus cause a high frequency current to flow through an induction heating coil provided at an inverter to generate a high frequency alternating magnetic field, thereby induction heating a load to be heated, such as an iron cooking iron utensil disposed in proximity to the induction heating coil for cooking the load or foods in the utensil.
In a case where an object to be heated, i.e., the load, is unsuitable, in other words, a smaller object, such as a spoon or a knife, or a pan of special stainless steel (18-8), is placed in proximity to the induction heating coil, an unsuitable load detecting circuit is required for deenergizing the induction heater by the reason that such low impedance load is unsuitable.
The unsuitable load detecting circuit has been well-known (disclosed in the Japanese Patent Publication No. 53-44061 (1978)) which is deenergized by the reason that when the load is a smaller object, an input power does not reach a certain percentage of the set value as compared with an input alternating current. Such apparatus ensures the unsuitable load detection when the setting input power is high; however, differences according to the load are smaller when the input power is low, whereby a reliable detection is difficult. Next, the reason for the above will be explained in accordance with FIG. 1.
FIG. 1 is a graph showing a relationship between the setting value of input power (on the abscissa axis) and the input AC power (on the ordinate axis) with regard to four kinds of loads, in which .circle.1 represents the characteristic of a porcelain enameled pan of 200 mm in diameter, .circle.2 represents that of a porcelain enameled pan of 120 mm in diameter, .circle.3 represents that of a pan of stainless steel (18-8) of 110 mm in diameter and 1.1 mm in thickness, and .circle.4 represents that of a pan of stainless steel (18-8) of 200 mm in diameter and 2.0 mm in thickness. Since the porcelain enameled pan represented by .circle.2 is smaller in diameter, the equivalent inductance viewed from an inverter circuit of the induction heating coil giving such load is larger to result in a large resonance frequency. Hence, when the input setting value is larger, the compensating operation of a frequency limit circuit makes it impossible to obtain an input coincident with the setting value. Since characteristics of the pans of stainless steel (18-8) represented by the lines .circle.3 and .circle.4 are smaller in a resistance component in the equivalent impedance of the induction heating coil giving such loads, an input lower than that of the porcelain enameled pan is obtainable.
As seen from FIG. 1, in a case when the pan of stainless steel (18-8) is intended to be detected as an unsuitable load, a threshold level for determining that the load is a smaller object, is set between the bent lines .circle.2 and .circle.3 , but in the low setting value region, the characteristics are close to each other, so that it is impossible to recognize the stainless steel pan as the unsuitable one. Since there also is the characteristic of the bent line as shown in .circle.2 , in order to decide whether or not the load .circle.2 is suitable, it is necessary to previously set the threshold level to be a bent line similar to the line .circle.2 , whereby complicated circuitry is inevitable. The input power change follows variations of the source voltage, thereby creating the problem in that the reliable detection is difficult.